(1) Field of the Invention
The invention relates to the general field of laser annealing with particular attention to the problem of shielding the laser system from laser emitted debris.
(2) Description of the Prior Art
As part of the manufacture of integrated circuits and similar devices, a need may arise to recrystallize an amorphous layer, either in its entirety or, more likely, in selected areas only. This can conveniently be accomplished by means of a laser beam which can be directed to scan an entire area or may be limited to selected areas as desired. Since no diffusion is involved, recrystallization will take place very rapidly once a sufficiently high temperature has been reached. Excimer lasers, which provide very high energy densities during short duration pulses, are well suited for this because such brief, high energy pulses allow small areas to be rapidly heated to high temperatures with little effect on the immediate surroundings.
One side effect of this high concentration of energy, even though the time be brief, is that the surface being irradiated by the laser beam tends to emit debris in the form of evaporant and/or finely divided particulate matter. This is schematically illustrated in FIG. 1 where laser beam 1 is seen to be directed to shine on layer 3 on substrate 4. As a side effect of the beam's interaction with layer 3 the afore-mentioned debris, shown schematically as arrows 5, is ejected. Some of this debris ends up on the underside of window 2. The latter, which is obviously transparent to the laser radiation being used, is needed either to protect the laser optics directly or, more commonly, because the layer 3 is inside an enclosure that may be evacuated or filled with an inert gas, so window 2 forms part of the top side of such an enclosure.
If the underside of window 2 is unprotected, the accumulation of laser debris on its underside will soon render it unsuitable for its intended purpose because it will become opaque to the laser radiation and/or will lose its optical flatness and smoothness. Thus, a number of approaches to protecting the laser admission window may be found in the prior art. For example, Carter et al. (U.S. Pat. No. 4,897,520 January 1990) describe a vacuum scoop that removes the debris as it is generated. This approach is clearly unsuitable for use in a vacuum environment. Offord et al. (U.S. Pat. No. H1637 March 1997) minimize the emission of debris by protecting all surfaces not intended for laser irradiation with a layer of aluminum which reflects the beam away. This method is not suitable for processes in which large areas are required to interact with the laser.
Flint (U.S. Pat. No. 4,439,259 March 1984) uses a disposable shield made from epoxy and teaches how to make such a shield, including a proper mount for it, at low cost but with the desired optical quality needed for sensitive laser operations. Ichinowkawa (U.S. Pat. No. 5,153,607 October 1992) describes a laser shutter mechanism that is interlocked to close whenever a laser printer is opened, as the laser may have been inadvertently left on.
Takenouchi et al. (U.S. Pat. No. 5,561,081 October 1996) describe the use of a laser for annealing semiconductor layers. They minimize the amount of emitted debris by preheating their films and by using a laser beam that has a rectangular cross-section similar to the beam shown in FIG. 1.
The present invention take a different approach from all of the above and describes a shielding system that is long lasting but nevertheless highly effective.